This invention relates to a machine employed to electrically cut a workpiece using a wire electrode.
The use of electrical energy for machining workpieces is widely known and employed and, as one of the latest techniques attracting attention, there is wire cutting electric discharge device which employs a wire-shaped electrode to electrically machine a workpiece as if the workpiece were machined by a fretsaw.
FIG. 1 is a constructional view illustrating the principles upon which the machine for implementing the above is based. In FIG. 1, a workpiece 1 is installed opposite to a wire electrode 2 via insulating liquid 3. The insulating liquid 3 is hereinafter referred to as the working liquid. The working liquid is jetted from a nozzle 6 across the gap between the workpiece 1 and the wire electrode 2 through a tank 4 and a pump 5. Relative movement of the workpiece 1 and the wire electrode 2 is carried out by a moving table 11 on which the workpiece 1 is mounted. The table 11 is driven by a Y-axis driving motor 13 and an X-axis driving motor 12. According to this arrangement, relative movement of the workpiece 1 and the electrode 2 corresponds to two-dimensional planar movement in terms of the aforementioned X, Y axis planes. The wire electrode 2 is supplied from a wire supply reel 7, is passed through a lower wire guide 8A and the workpiece 1, reaches an upper guide 8B, and is then wound on a wire reel also serving as a tension roller 10 via an electric energy supply assembly. The operating power supply 15 for supplying the electrical energy is quite similar to that shown in the example illustrated herein and is composed of a d.c. power supply 16, a switching element 17, a current limiting resistor 19 and a control circuit 20 for controlling the switching element 17. There is also shown a control unit 14 for driving and controlling the X, Y axis motors 12, 13 and, as parts thereof, there are device controlling units, copying units and computer-controlled units which are of ordinary types.
The operation of the conventional wire cut electric discharge machine is as follows:
In a normal machining condition, a high frequency pulse voltage is applied by the operating power supply 15 and part of the workpiece 1 is fused and scattered with the discharge explosion caused by one pulse. Since the liquid in the interpole gap is gasified and ionized because of the high temperature, a certain period of quiescent time is required until the next pulse voltage is applied. If this quiescent time is too short, the interpole gap will not yet be again sufficiently insulated. As a result, a concentration of electric discharges in the same place for a second time causes the fusing of the wire electrode 2.
Accordingly, it is common in the case of an ordinary operating power supply to set up electrical conditions including the quiescent time of the operating power supply 15 depending on the kind of workpiece and plate thickness, and to carry out electric discharge machining with such electrical conditions as are more than necessary to prevent the wire electrode from being cut. Consequently, the machining speed will inevitably be reduced to a value considerably lower than the logical threshold value, and the fusing of the wire electrode 2 is inescapable should there be an inequality of the wire electrode diameter or any projection thereon or flaw thereof.
As noted above, the disadvantage of the conventional wire cut electric discharge machine is that the machining speed is extremely low because the output energy of the operating power supply is reduced to prevent the wire electrode 2 from being fused even if electric discharges are concentrated in one place.